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An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the amount of life on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the rate of speciation. Because the majority of diversity and biomass on Earth is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total diversity and abundance of life. Extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organisms. The Great Oxygenation Event was probably the first major extinction event. Since the Cambrian explosion five further major mass extinctions have significantly exceeded the background extinction rate. The most recent and debatably best-known, the Cretaceous–Paleogene extinction event, which occurred approximately million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. In addition to the five major mass extinctions, there are numerous minor ones as well and the ongoing mass-extinction caused by human activity is sometimes called the sixth extinction. Mass extinctions seem to be a Phanerozoic phenomenon, with extinction rates low before large complex organisms arose. Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity. ==Major extinction events== In a landmark paper published in 1982, Jack Sepkoski and David M. Raup identified five mass extinctions. They were originally identified as outliers to a general trend of decreasing extinction rates during the Phanerozoic,〔 but as more stringent statistical tests have been applied to the accumulating data, the "Big Five" cannot be so clearly defined, but rather appear to represent the largest (or some of the largest) of a relatively smooth continuum of extinction events.〔 # Cretaceous–Paleogene extinction event (End Cretaceous, K-Pg extinction, or formerly K-T extinction): Ma at the Cretaceous (Maastrichtian)-Paleogene (Danian) transition interval. The event formerly called the Cretaceous-Tertiary or K–T extinction or K-T boundary is now officially named the Cretaceous–Paleogene (or K–Pg) extinction event. About 17% of all families, 50% of all genera〔 and 75% of all species became extinct. In the seas it reduced the percentage of sessile animals (those unable to move about) to about 33%. All non-avian dinosaurs became extinct during that time. The boundary event was severe with a significant amount of variability in the rate of extinction between and among different clades. Mammals and birds, the latter descended from theropod dinosaurs, emerged as dominant large land animals. # Triassic–Jurassic extinction event (End Triassic): 201.3 Ma at the Triassic-Jurassic transition. About 23% of all families, 48% of all genera (20% of marine families and 55% of marine genera) and 70% to 75% of all species went extinct.〔 Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition. Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments. The Temnospondyl lineage of large amphibians also survived until the Cretaceous in Australia (e.g., ''Koolasuchus''). # Permian–Triassic extinction event (End Permian): 252 Ma at the Permian-Triassic transition. Earth's largest extinction killed 57% of all families, 83% of all genera and 90% to 96% of all species〔 (53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species, including insects).〔 〕 The highly successful marine arthropod, the trilobite became extinct. The evidence of plants is less clear, but new taxa became dominant after the extinction. The "Great Dying" had enormous evolutionary significance: on land, it ended the primacy of mammal-like reptiles. The recovery of vertebrates took 30 million years, but the vacant niches created the opportunity for archosaurs to become ascendant. In the seas, the percentage of animals that were sessile dropped from 67% to 50%. The whole late Permian was a difficult time for at least marine life, even before the "Great Dying". # Late Devonian extinction: 375–360 Ma near the Devonian-Carboniferous transition. At the end of the Frasnian Age in the later part(s) of the Devonian Period, a prolonged series of extinctions eliminated about 19% of all families, 50% of all genera〔 and 70% of all species. This extinction event lasted perhaps as long as 20 million years, and there is evidence for a series of extinction pulses within this period. # Ordovician–Silurian extinction events (End Ordovician or O-S): 450–440 Ma at the Ordovician-Silurian transition. Two events occurred that killed off 27% of all families, 57% of all genera and 60% to 70% of all species.〔(【引用サイトリンク】title=extinction )〕 Together they are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct. Despite the popularization of these five events, there is no fine line separating them from other extinction events; using different methods of calculating an extinction's impact can lead to other events featuring in the top five. The older the fossil record gets, the more difficult it is to read. This is because: * Older fossils are harder to find as they are usually buried at a considerable depth. * Dating older fossils is more difficult. * Productive fossil beds are researched more than unproductive ones, therefore leaving certain periods unresearched. * Prehistoric environmental events can disturb the deposition process. * The preservation of fossils varies on land, but marine fossils tend to be better preserved than their sought after land-based counterparts.〔Sole, R.V., and Newman, M., 2002. "Extinctions and Biodiversity in the Fossil Record – Volume Two, ''The Earth system: biological and ecological dimensions of global environment change''" pp. 297–391, ''Encyclopedia of Global Environmental Change'' John Wilely & Sons.〕 It has been suggested that the apparent variations in marine biodiversity may actually be an artifact, with abundance estimates directly related to quantity of rock available for sampling from different time periods. However, statistical analysis shows that this can only account for 50% of the observed pattern, and other evidence (such as fungal spikes) provides reassurance that most widely accepted extinction events are real. A quantification of the rock exposure of Western Europe indicates that many of the minor events for which a biological explanation has been sought are most readily explained by sampling bias. Research completed after the seminal 1982 paper has concluded that a sixth mass extinction event is ongoing: :6. Holocene extinction: Currently ongoing. Extinctions have occurred at over 100 times the background extinction rate since 1900. The mass extinction is considered a result of human activity.〔(It's Official: A Global Mass Extinction is Under Way - James Macdonald - JSTOR Daily )〕〔(We're Entering A Sixth Mass Extinction, And It's Our Fault | Grennan Milliken | Popular Science )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「extinction event」の詳細全文を読む スポンサード リンク
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